Field of the Invention
The invention relates to a method for firing a firing cap of a vehicle occupant protection system of a motor vehicle. Furthermore, the invention is directed at a firing circuit of the vehicle occupant protection system of the motor vehicle. The firing circuit contains a firing switch connected in series with the firing cap and can be actuated by a firing pulse. The circuit further contains at least two capacitors for energizing the firing cap.
Because the vehicle occupant protection system of the motor vehicle must reliably trigger in the event of an accident (crash) even if a connection to the vehicle electrical system happens to be interrupted, one or more firing capacitors are usually provided. The firing capacitors are charged during normal operation of the motor vehicle and make available the power that is necessary to fire the firing cap or caps of the vehicle occupant protection system. The firing caps are connected to the firing capacitors via a firing switch that can be provided with a current limiter. Because a minimum voltage has to be maintained at the firing cap during the entire firing period in order to ensure that there is a current which does not drop below a specific minimum value, a relatively large part of the power usually remains stored uselessly in the capacitors. The quantity of power that remains must be greater here than the quantity of power consumed. Therefore, relatively large firing capacitors with a high capacitance value and high charge value have to be made available which require large amounts of space and therefore constitute an obstacle to a more compact configuration of the firing circuit. Because the firing caps have to be/are to be fed with a constant current, a current limiter, which consumes a large part of the power, is used.
Published, Non-Prosecuted German Patent Application DE 44 09 019 A1 discloses a firing circuit for a vehicle occupant protection device in a motor vehicle in which two firing capacitors make available sufficient power. Here, only the first firing capacitor is discharged within a first time period starting from the firing before only the second firing capacitor is discharged after a defined time period starting from the firing.
International Patent Disclosure WO 93/17893 discloses a method for triggering a vehicle occupant protection system and a firing circuit. In the known firing circuit, two or more firing capacitors are provided which are connected in parallel with one another and with the motor vehicle battery during normal operation of the motor vehicle, with the result they are each charged to the voltage of the vehicle electrical system. The object of this publication is, in the event of an accident, to supply the firing cap immediately with a voltage that lies significantly above the voltage of the vehicle electrical system, so that the triggering process can reliably start. For this purpose when an accident is sensed, the firing capacitors are immediately connected in series and the firing switch is closed so that the firing cap is supplied with a voltage which is twice as high as the voltage of the vehicle electrical system or is even higher when a plurality of capacitors are used.